1. Field of the Invention
The present invention relates to distance measuring finders and, more particularly, to a distance measuring finder combining a distance measuring device based on the principle of the base line distance meter with a mark finder.
2. Description of the Prior Art
In general, a distance measuring device based on the principle of the base line distance meter is one that performs distance measuring operation by projecting infrared light from a light emitting device onto an object being photographed through a projection optical system and by causing the reflected light from said object to be received by a light receiving device through a light collecting optical system. This distance measuring operation of the distance measuring device is divided into two major types. One type uses a plurality of light receiving elements as the light receiving device, and the other type uses a single light receiving element.
The type using the plurality of light receiving elements selects the light receiving elements in sequence as the photographic lens is focused. Upon detection of the one light receiving element which receives most of the reflected light from the object, the output of that light receiving element functions as an in-focus signal to stop the focusing operation of the photographic lens. On the other hand, the type using the single light receiving element scans the object by making movable either the light receiving element or the light emitting device in response to the focusing operation of the photographic lens. Upon the detection of the instant at which most of the reflected light from the object is received, the concurrent output of the light receiving element functions as the in-focus signal to stop focusing operation of the photographic lens.
Such distance measuring device of whichever type can be easily improved in the accuracy of distance measurement by increasing the interval between the projection optical system and the light collecting optical system. This may be done providing that the output of the light emitting device, the sensitivity of the light receiving device, the accuracy of the signal processing circuit for the light receiving device, and the like are constant.
When the interval between the projection optical system and the light collecting optical system is made too large, however, the size of the camera increases. This results in making the camera less portable. For this reason, it is necessary to limit the interval between the projection optical system and the light collecting optical system to a predetermined one necessary for maximum portability of the camera.
For this reason, according to the prior art, by providing a finder of a construction so simple that an objective lens and an eyepiece are only arranged on a common optical axis between the projection optical system and the light collecting optical system of the distance measuring device as in the Albata finder, the creation of a necessary predetermined interval between the projection optical system and the light collecting optical system was made possible; the space between the projection optical system and the light collecting optical system being effectively utilized.
As far as the finder is concerned, however, since the finder is of such simple construction that the objective lens and eyepiece are only arranged on the common optical axis, it is difficult to display a wide variety of information in the finder field, and even to display the range of measurement of distances. In particular, since, recently, it has become of great importance to display such information in view of camera automation, such finder cannot be said to be satisfactory in practice.
For this reason, a mark finder capable of displaying information with ease may be combined with the distance measuring device. Since the mark finder requires the provision of a field framing plate in parallel with the objective lens, however, the arrangement of such mark finder between the projection optical system and the light collecting optical system of the distance measuring device calls for an unduly large increase in the size of the camera. The result is a disadvantage in that the portability of the camera is largely reduced.
An object of the present invention is to solve the above-mentioned drawbacks and to make it possible to combine a distance measuring device based on the principle of the base line distance meter with a mark finder without causing the size of the camera to be increased.
Another object of the present invention is to make it possible that the range of measurement of distances is clearly recognized in the view field.